Last time: is a population evolving?
- Re-examine the beetle populations
- color: one locus, 2 alleles
- Across 2 generations, are allele frequencies the same?

Hardy Weinberg describes a population NOT evolving

Allele frequencies change in beetles: Mutation
- Mutations in the beetle color gene, resulting in more brown
- to rare to be considered evolutionary change
- Mutation rate =
- mutation rates are generally small (~0.0001%).
- humans = 2.5 x 10-8 mutations nucleotide-1 generation-1
- negligible change from one generation to the next
- Mutation may counterbalance selection
- selection against a deleterious recessive allele (a)
- but mutation from A -> a
- selection may not completely eliminate the ‘a’ allele

Allele frequencies change in beetles: Genetic Drift
- Genetic drift describes how allele frequencies fluctuate unpredictably across generations
- happens in every population
- pronounced in small populations
- extremely important evolutionary “force”
- Reduce genetic variation through the random loss of alleles
- In a small population, beetles are green or brown
- several green beetles get stepped on by Dr. C
- next generation has more brown beetles, by chance
- did it matter if alleles were dominant or recessive?

Coin flips to choose 2 alleles

Mechanism of Genetic Drift: Founder Effect
- Founder Effect: when a few individuals become isolated from a larger population
- some alleles make it, others lost
- non-random
- Occurs frequently on islands
- what could happen with bad alleles?



Mechanism of Genetic Drift: Bottleneck Effect
- Bottlenecks occurs after a drastic reduction in population size
- sudden change in the environment
- The new gene pool may be different from original population
- If the population stays small, genetic drift could increase

Bottlenecks and species habitat loss

Allele frequencies change in beetles: Gene Flow

- Gene flow: exchange of alleles between 2 or more populations
- aka ‘Gene migration’
- like mutation but way more powerful
- Occurs when new individual migrates and mates successfully
- brown beetles enters population
- changes allele frequency for color
- Gene flow can be good or bad
- may increase genetic diversity
- bring harmful alleles

Allele frequencies change in beetles: Natural Selection

Only natural selection causes adaptive evolution!
- ‘Sorting’ of alleles that favor reproductive success
- Certain traits lead to greater Relative Fitness
- individuals with good trait will contribute more to gene pool
- better survival/reproduction compared to others
- More individuals in a population with greater ‘RF’ means….
- Natural selection is a ‘moving target’

Natural Selection: selection of heritable traits




